1. Field of the Invention
The present invention relates to an apparatus for ionizing and vaporizing relatively nonvolatile or thermally labile compounds for analysis in a mass spectrometer.
2. Description of the Prior Art
In the analysis of compounds by mass spectrometry, it is necessary to ionize and vaporize the compound to be analyzed prior to passage through the mass spectrometer analyzer no matter how relatively nonvolatile the compound to be analyzed actually is. In the past, such techniques as electron impact (EI), field ionization (FI) and chemical ionization (CI) mass spectrometry have been employed for the structural analysis of salts and highly polar, thermally labile, organic molecules. However, the use of these techniques for the analysis of such types of compounds has been severely limited by the requirement that the compound to be analyzed must be in the gaseous state prior to ionization. For compounds of the above type, frequently the energy required to disrupt the bonding between contiguous molecules of the sample in the solid state or the energy between the sample molecules and the surface of the sample holder often is in excess of the energy necessary to break intramolecular bonds within the sample molecules. Thus, these techniques frequently result in substantial decomposition of sample molecules before they can undergo ionization and vaporization. Moreover, the energy from the device used to heat the sample molecules is distributed in many internal degrees of freedom in the molecules and as a result, the competition between degradation of the sample molecules by dissociation of intramolecular bonds and disruption of surface-sample and/or sample-sample interactions is dominated by the molecular degradation process.
Recently, advances have been made in the development of techniques which enhance the ionization-vaporization of sample molecules for analysis by mass spectrometry while minimizing the rate of decomposition of the molecules. In one technique, a ultra rapid heating technique is used to volatilize molecules deposited on a nickel foil. Heating is accomplished by the impact of 200 Mev .sup.252 CF fission fragments. By this technique many high molecular weight biological molecules including Vitamin B.sub.12 have been analyzed. In another technique, strong electrostatic fields have been used to promote the ionization of polymeric, nonvolatile, or thermally labile organic molecules which are sprayed into an ion source in the form of organic solutions.
U.S. Pat. No. 3,555,272 shows a chemical ionization technique for the generation of gaseous ions from a sample in which a first gaseous reactant material and a second gaseous material which is the sample material to be analyzed are introduced into the ionization chamber of the spectrometer. The mixture of gases predominantly contains the first gaseous reactant. The gaseous mixture is then subjected to ionizing conditions to form stable ions of the gaseous reactant, which subsequently react by ion-molecule interactions with the molecules of the gaseous sample under the pressure and ionizing conditions in the ionizing chamber thereby producing gaseous ions characteristic of the sample molecules. The ionized gases are then introduced into the mass spectrometer analyzer. It is evident from the above description of the method of U.S. Pat. No. 3,555,272 that the technique described is limited only to mixtures of gaseous molecules and gaseous ions.
In still another technique, known as field desorption (FD) mass spectrometry, a strong electrostatic field is set up between a plate electrode which functions as a cathode and an anode upon which is deposited the material to be ionized and vaporized. The cathode also contains a port through which gaseous ions pass into the mass spectrometer analyzer. When the electrostatic field is generated, sample molecules suffer ionization and are desorbed as ions into the gaseous state. The anode or emitter is formed of a 10 .mu.m tungsten wire covered with a large number of carbon microneedles which are about 30 .mu.m in length. The potential difference between the anode and cathode plate is on the order or 10,000 volts and the chamber which houses the sample is under a high vacuum of 10.sup.-5 to 10.sup.-7 torr. The above potential produces a field at the emitter which is on the order of 1 V/A. The anode or emitter is heated by a flowing current until sample ions are observed on the mass spectrometer recorder. It is believed that ionization of the molecules coated on the emitter anode occurs by the combined effect of thermal energy and the applied field. In other words, electrons from sample molecules tunnel through to the emitter wire and the resulting coulombic repulsion expels the ions from the emitter. The ions enter the gaseous phase and traverse the mass analyzer part of the spectrometer. The principal features of the field desorption technique are the use of (1) an activated surface (emitter), i.e., an anode formed by the deposition of carbonaceous dendrites on thin tungsten wire or in a more recent embodiment of such an anode, rough metal surfaces formed by breaking a brittle metal (tungsten) rod of 1 mm (OD) or by electrochemical processes, and (2) a high field on the order of 10,000 volts (1 V/A). However, a disadvantage of the field desorption technique is that the high electrostatic potentials required prevent the technique from being used under chemical ionization (CI) conditions when the ion source is filled with a reagent gas at a pressure on the order of 0.1-1 torr. At these pressures in the ionization chamber, most gases conduct electric current and severe arcing occurs if a 10,000 volt field is present. The high field requirement renders it extremely difficult to use the field desorption technique for sample ion production in quadrupole mass spectrometers. Under FD conditions ions are expelled from the emitter with energies on the order of 10,000 volts. Since the quadrupole mass filter only functions efficiently when the ion energy is in the range of 0-40 volts, the FD ion source must be modified substantially in order to decelerate the ions to velocities compatible with the requirements of the quadrupole mass filter.
A need, therefore, continues to exist for a field desorption technique which can be used satisfactorily for ion generation and introduction at relatively low energies and velocities such as are required for quadrupole mass spectrometers.